1. Field of the Invention
This invention relates to a drive circuit for a brushless DC motor, and more particularly to such a drive circuit in which objectionable audible sound generated in a stator coil of the motor is substantially reduced.
2. Description of the Prior Art
In a switching type brushless DC motor, transistors are conventionally coupled to armature coils mounted on the stator of the motor. Rectangular drive signals are applied to these transistors to switch them over so that they alternately supply current to the armature coils. Since the drive signal is a rectangular wave, it includes a significant proportion of high frequency components. When these high frequency components reach the armature coil, the latter acts like the voice coil of a speaker, and an objectionable electromagnetic noise appears, usually as an audible hum. This electromagnetic noise can be bothersome, particularly when the load to the brushless DC motor is large. In such circumstances, the current flowing through the armature coil is large, and the electromagnetic sound from the armature coil becomes loud and distracting.
One conventional technique to reduce the electromagnetic sound involves applying an approximately sinusoidal wave as the drive signal. When a sinusoidal wave is so used, current should be supplied to the armature coil both on the positive side of the sinusoidal wave and also on the negative side thereof. Accordingly, two drive transistors are required for each armature coil, and positive and negative power sources are required to generate the sinusoidal wave drive signal. Consequently with such a technique the number of circuit elements required is disadvantageously large, and motor efficiency is low.
In another technique, the rectangular drive signal is smoothed in an RC circuit, i.e., a filter circuit including a time constant circuit formed of a capacitor and a resistor. In this technique, a rectangular wave is smoothed or dulled in the RC filter circuit so that the drive signal is correspondingly smoothed. The rising time of the drive signal is delayed by a time t.sub.1, and its falling time is delayed by a time t.sub.2 from the corresponding switching times of the rectangular wave, and this delay decreases motor efficiency. In particular, when the rotational speed of the motor is high, that is, when the width of the rectangular wave is small, the ratio of the delay times t.sub.1 and t.sub.2 to the width of the rectangular wave becomes large. Although the delay times can be decreased by lowering the RC time constant of the filter circuit to promote efficiency, the electromagnetic sound from the armature coil becomes louder as the RC time constant decreases. Accordingly, the time constant of the filter circuit is conventionally selected to be large enough that inefficiency results at high speeds.